St. James, I don't know about sage advice, but here are my thoughts on the subject.
I am a new shooter, just like you. I've been at it a little longer though, a couple of months as opposed to your couple of weeks.

It's actually a bit scary how similar you and I are in our firearm choices. I too chose the 22/45 as I thought that it was the gun that would make the most sense for me learning the fundamentals. The .22 is an inexpensive caliber and one that I can easily afford to shoot for several hours. My research indicates that learning the three basic principles of sight picture, trigger pull, and breathing control as well as proper gun handling techniques is learned just as effectively on the .22 as it is with a bigger caliber.
The reason I chose the 22/45 over other available .22s is that its controls and grip angle roughly approximate the 1911 platform, my handgun of choice. It's not the same of course, the Ruger is thinner and the safety is a little different, but the magazine release and safety are located in the same place. Still, they're close enough that I find the 22/45 a very good trainer.
Keep in mind though that the grip of the 22/45 is made of plastic, unlike the all metal construction of the rest of the Ruger Mark pistols. It's not an issue for me, I just thought I'd mention it. As to whether or not you should get the Mark III or the older Mark II, I guess that's up to you. I personally don't have much experience with firearms, so I went with the Mark III so that I would have the peace of mind that comes with having a new firearm. You have to remember that one of the biggest advantages to the .22 is cheap ammo. Any used .22 will likely have thousands and thousands of rounds shot through it. By the time you get a used gun it could be worn out. Plus, the 22/45 is already an inexpensive firearm when new, it's less than the cost of a new Sebenza. I say skip the possible headache and just get a new one.
Don't worry about the disassembly and reassembly of the pistol. The Ruger 22/45 was my first pistol and when I field stripped it that was the first time I ever took apart a firearm. It was a bit difficult at first, but as long as you follow the directions in the manual you'll be fine. I feel that field stripping the 1911 is more difficult.
BTW, are you still in CA? If you still make it out to the Bay Area regularly I have a great gun shop I can recommend to you. The guys are incredibly friendly and helpful, and they showed me how to detail strip my Kimber Custom II when I bought it.